I did my car when it was on a lift. I used a rag dipped in a pan of gasoline and a whole lot of elbow grease. Kerosene also works, but gas is easier to obtain and a bit cheaper as well. You have to do this in a well ventilated area. The gas loosens up the undercoating/sound deadener like crazy, and I used a fairly small screwdriver to get into tight corners once the material was softened up. The gas evaporates relatively quickly, leaving the original paint (and rust!) unharmed. I wanted to preserve whatever was there before taking a paint stripper or stripping wheel to the damaged areas.